Hybrid fiber tows containning both nano-fillers and continuous fibers, hybrid composites, and their production processes

ABSTRACT

Disclosed is a hybrid fiber tow that comprises multiple continuous filaments and nanoscale fillers embedded in the interstitial spaces between continuous filaments. Nanoscale fillers may be selected from a nanoscale graphene plate, non-graphite platelet, carbon nano-tube, nano-rod, carbon nano-fiber, non-carbon nano-fiber, or a combination thereof. Also disclosed are a hybrid fiber tow impregnated with a matrix material and a composite structure fabricated from a hybrid fiber tow. The composite exhibits improved physical properties (e.g., thermal conductivity) in a direction transverse to the continuous fiber axis. A roll-to-roll process for producing a continuous fiber tow or matrix-impregnated fiber tow and an automated process for producing composite structures containing both continuous filaments and nanoscale fillers are also provided.

This invention is based on the research results of a project supported by the U.S. Department of Energy (DOE) SBIR-STTR Program. The US Government has certain rights on this invention.

FIELD OF INVENTION

The present invention is related to a hybrid composite containing both a nano-filler and a continuous fiber dispersed in a matrix material. The nano-filler comprises nano-scaled graphene plates (NGPs), carbon nano-tubes (CNTs), carbon nano-fibers (CNFs), nano-clay platelets, nano-rods, or any other nanoscale reinforcement with at least an elongate axis. The matrix material comprises a polymer, organic, metal, ceramic, glass, carbon material, or a combination thereof. The nano-filler can be made to be substantially oriented in a preferred direction (e.g., with an elongate axis perpendicular to the continuous fiber).

BACKGROUND

Advanced composites, containing continuous fibers dispersed in a matrix material, are widely used in aerospace, sports equipment, infrastructure, automotive, and other transportation industries, as both primary and secondary load-bearing structures. These composite materials derive their excellent mechanical strength, stiffness, electrical conductivity, and thermal conductivity from the reinforcement fibers. However, using polymer matrices and carbon or graphite fibers are examples, continuous carbon fiber reinforced polymer composites exhibit these good properties only in the directions parallel to the fiber axial directions. In other words, these composites have excellent in-plane properties and relatively poor thickness-direction and shear properties.

Specifically, the thickness-direction and shear strengths and moduli of continuous carbon fiber reinforced polymer composite laminates are relatively poor. Poor interlaminar shear strengths in turn lead to poor delamination resistance. Incorporation of reinforcements that are oriented in a direction perpendicular to the continuous fiber axis can significantly improve these mechanical properties.

In addition, the longitudinal conductivities (both thermal and electrical) of a carbon fiber are orders of magnitude greater than its corresponding transverse conductivities. Hence, the transverse conductivities of a composite laminate are also much lower than the longitudinal properties. The transverse conductivities of a continuous fiber composite can be significantly improved by incorporating a reinforcement phase perpendicular to the continuous fiber axis. However, the addition of transverse reinforcements such as short (chopped) fibers is known to create processing difficulties. Even with nanoscale fillers like carbon nano-tubes (CNTs) and carbon nano-fibers (CNFs), a small amount of these nano-fillers could dramatically alter the flow properties (e.g., increased viscosity) of a matrix resin. The resulting matrix material, a nano-filler/resin mixture, is typically so viscous that it becomes extremely difficult to disperse continuous fibers in this matrix.

Further specifically, on the one hand, the loading of conductive nano-fillers (e.g., >5 wt. % of CNTs or CNFs) increases the viscosity of the matrix resin to a level that is not conducive to filament winding and other automated composite manufacturing techniques. On the other hand, a low percentage (<5% by wt.) of nano-fillers normally does not provide adequate through-thickness thermal or electrical conductivity to a composite structure for certain engineering applications. A need exists to develop an approach that can resolve these conflicting issues; i.e. a process capable of combining continuous fibers with a matrix that features an adequate proportion of nano-fillers dispersed in a resin or other material with these fillers preferentially oriented along a desired direction for improved transverse or shear properties.

Generally, the advantages of nanoscale reinforcements in polymer matrices are fourfold: (1) when nanoscale fillers are finely dispersed in the matrix, the tremendous surface area developed could contribute to both polymer chain confinement and load transfer effects, leading to higher glass transition temperature, stiffness, and strength; (2) nanoscale fillers provide an extraordinarily zigzagging, tortuous path that leads to enhanced resistance to micro-cracking; (3) nanoscale fillers can also enhance the electrical and thermal conductivities; and (4) carbon-based nanoscale fillers have excellent thermal protection properties and, when incorporated in a matrix material, could eliminate the need for a thermal protective layer—for instance, in missile and rocket applications.

Fabrication of carbon nanotubes (CNTs) is expensive, particularly for the purifying process required to make them useful in applications. Instead of trying to discover lower cost processes for CNTs, we have been seeking to develop an alternative nanoscale carbon material with comparable properties that can be produced cost-effectively and in larger quantities. This development work has led to the discovery of processes for producing a new class of nano material herein referred to as nanoscale graphene plates (NGP) [Jang, et al., “Nano-scaled Graphene Plates,” U.S. Pat. No. 7,071,258 (Jul. 4, 2006) and “Process for Producing Nano-scaled Graphene Plates,” U.S. patent pending, Ser. No. 10/858,814 (Jun. 3, 2004)]. An NGP is a nanoscale platelet composed of one or more layers of graphene plane. In a graphene plane, carbon atoms occupy a 2-D hexagonal lattice. These carbon atoms are bonded together through strong covalent bonds lying on this plane. In the c-axis direction, several graphene planes may be weakly bonded together primarily through van der Waals forces. An NGP may be viewed as a flattened sheet of a CNT. Although NGP and CNT are geometrically different in architecture, preliminary calculations have indicated very similar mechanical properties (in-plane stiffness and strength) and thermal and electrical conductivities (Table 1).

TABLE 1 Estimated physical constants of CNTs and NGPs. Property Single-Walled CNTs NGP Specific Gravity 0.8 g/cm³ 1.8–2.2 g/cm³ Elastic modulus ~1 TPa ~1 TPa (in-plane) Strength 50–500 GPa ~100–400 GPa Resistivity 5–50 μΩ cm 50 μΩ cm (in plane) Thermal Up to 1,500 W m⁻¹ K⁻¹ 3,000 W m⁻¹ K⁻¹ (in-plane) Conductivity (estimated) 6–30 W m⁻¹ K⁻¹ (c-axis) Magnetic 22 × 10⁶ EMU/g (⊥ to 22 × 10⁶ EMU/g (⊥ to the Susceptibility the plane) plane) 0.5 × 10⁶ EMU/g (|| to 0.5 × 10⁶ EMU/g (|| to the the plane) plane) Thermal Negligible (theoretical) −1 × 10⁻⁶ K⁻¹ (in-plane) expansion 29 × 10⁻⁶ K⁻¹ (c-axis) Thermal stability >700° C. (in air); 450–650° C. (in air) 2800° C. (in vacuum) Specific surface Typically 100–500 m²/g Up to 1,500 m²/g area

NGP-reinforced composites are also expected to exhibit similar properties compared to CNT-reinforced composites. When the NGP-resin mixture, a nanocomposite, is incorporated as a matrix for forming a continuous fiber reinforced composite, the resulting hybrid composite (containing both the NGP and the continuous fiber as reinforcement phases) is expected to have improved mechanical and physical properties compared to the conventional fiber composite (containing only continuous fiber, no NGP). A need exists for incorporating both NGPs and continuous fibers in a matrix material to make a hybrid composite. Further, as indicated earlier, the loading of nano-fillers (e.g., CNTs and CNFs) increases the viscosity of the matrix resin to a level that is not conducive to subsequent filament winding and other automated composite manufacturing techniques. A need exists to develop a process that is capable of combining both continuous fibers and CNTs, CNFs, other nano-rods, or nano-platelets with a matrix material to make a hybrid composite. Preferably, such a process can be automated.

SUMMARY OF THE INVENTION

The present invention provides a process that is capable of producing a hybrid composite that contains both nano-fillers (e.g., NGPs, CNTs, CNFs, or a combination thereof) and continuous fibers as reinforcement phases dispersed in a matrix material. The nano-fillers can be oriented in a direction that is non-parallel to the longitudinal axis direction of the continuous fiber (e.g., preferably perpendicular to the continuous fiber axis). The process begins with spreading a continuous fiber tow separate continuous filaments from each other and then incorporating nano-fillers in a continuous fiber tow with individual nano-fillers embedded in the interstitial spaces between continuous filaments. The resulting hybrid fiber tow is then impregnated with a resin to produce a resin-pre-impregnated hybrid tow or hybrid towpreg. This wet hybrid towpreg can then go through a filament winding, fiber placement, prepregging, or pultrusion process for making a composite structure.

Alternatively, the hybrid fiber tow can be woven, braided, knitted, or stitched into a textile-structured preform, which is then impregnated with a resin.

The process may involve incorporating continuous fibers, nano-fillers, and a matrix-making material in a powder form concurrently to form a matrix-forming powder-impregnated hybrid fiber tow, a dry hybrid towpreg. The dry hybrid towpreg can then go through a filament winding, fiber placement, weaving, braiding, knitting, or stitching to form a structured preform, which is then converted to become a hybrid composite structure by heating and consolidating the matrix-forming material to become the solid matrix material.

The present invention also provides hybrid fiber tows, hybrid fiber towpregs, and resulting hybrid fiber composites that can be composed of a wide range of fibers, fillers, and matrix materials.

The versatility of the invented process opens up a window of many application opportunities for hybrid composites containing continuous fibers and nano-fillers such as NGPs, CNTs, and CNFs.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 (a) Schematic of a hybrid fiber tow containing nano-fillers 16,18 embedded in the interstitial spaces 14 between continuous filaments 12 and (b) schematic of a hybrid fiber towpreg containing both nano-fillers and a matrix-forming material (e.g., thermoplastic powder particles 20) embedded in the interstitial spaces between continuous filaments.

FIG. 2 Schematic of a roll-to-roll process for continuous production of hybrid fiber tows or towpregs.

FIG. 3 Schematic of a process for continuously producing a towpreg and fabricating a composite structure.

FIG. 4 Schematic of another version of the roll-to-roll process for the continuous production of hybrid fiber tows or towpregs.

FIG. 5 Transverse thermal conductivity of hybrid composites containing continuous carbon fibers and NGPs (7 compositions) and CNTs (2 compositions).

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The conventional approach to fabricating composite materials containing both continuous fibers and fillers (such as nanoscale fillers, short fibers, etc.) typically involves mixing the fillers with a resin first, followed by impregnating the continuous fiber tows with the resin/filler mixture. It is now well-recognized that a small amount of nano-fillers like carbon nano-tubes (CNTs) and carbon nano-fibers (CNFs) could dramatically increase the viscosity of a matrix resin. The resulting nano-filler/resin mixture is typically so viscous that it becomes extremely difficult to disperse continuous fibers in this matrix. Hence, it is also commonly believed that only a small amount of nano-fillers can be incorporated in a hybrid composite.

Furthermore, the prior-art sequence of mixing nano-fillers with a resin and then impregnating continuous fibers with the nano-filler/resin mixture tends to produce a hybrid composite with fillers oriented along the continuous fiber axis. Such an orientation does not improve thickness-direction properties and shear properties of a composite laminate with continuous fibers lying on a laminar plane.

Contrary to what composite materials experts would or might expect, we have developed an approach that enables the fabrication of hybrid composites containing a high proportion of nano-fillers with a preferential orientation that is substantially perpendicular to the continuous fiber axis (FIG. 1( a)). Instead of mixing the nano-fillers with a matrix resin first and then impregnating continuous fibers with the nano-filler/resin mixture, we took a novel and innovative approach that involved mixing the nano-fillers with continuous fibers first to produce a hybrid fiber tow (with nano-fillers such as NGPs 16 and/or CNTs 18 embedded in the interstitial spaces 14 between continuous fibers 12) and then impregnating the hybrid fiber tow with a resin. An important step was to spread a continuous fiber tow into separated, individual continuous filaments prior to nano-filler mixing or addition. It turns out that, with this sequence, the resin can readily penetrate into the interstitial spaces and wet out both the continuous fibers and nano-fillers. Any resin that is commonly used in a filament winding, prepregging, fiber placement, or pultrusion operation can be used in the invented process. This process surprisingly but elegantly accomplishes the task of fabricating advanced composites containing both continuous filaments (fibers) and nano-fillers as reinforcement phases.

This process also enables impregnation of continuous fiber/nano-filler preform shape with a ceramic, glass, or carbon matrix via a specialized technique like chemical vapor infiltration to produce corresponding hybrid composites, which otherwise would be difficult to obtain.

As an example to illustrate this process, a fluidized-bed powder impregnation or coating process (FIG. 2) has been successfully adapted for incorporating a controlled percentage of nano-fillers (e.g., NGPs, CNFs, and/or CNTs) in a continuous fiber tow (consisting of multiple filaments) to produce a hybrid fiber tow containing nano-fillers residing in the interstices between individual filaments. The process begins with continuously feeding a fiber tow into a fiber tow spreader to separate individual carbon filaments from one another, opening up interstitial spaces between filaments to accommodate nano-fillers. The separated filaments are fed into a fluidized bed chamber in which nano-fillers are driven by an air flow to move around like a fluid. When the air-suspended fillers impinge upon the filaments with desired interstitial space sizes the nano-fillers are trapped in the interstitial spaces with an elongate axis of the filler typically oriented in the fluid flow direction. This direction can be controlled to ensure that a majority of the fillers are oriented perpendicular to the longitudinal axis of the continuous fibers. The resulting structure is a continuous hybrid fiber tow, as schematically shown in FIG. 1( a).

Electrostatic charges may be imparted to nano-fillers to facilitate attraction of nano-fillers to the carbon fiber tow. This is analogous to the conventional towpreg production operation by which micron-scaled thermoplastic powder particles, serving as a precursor to the composite matrix, are incorporated into a continuous fiber tow [e.g., J. D. Muzzy, et al., U.S. Pat. No. 5,094,883, Mar. 10, 1992]. No nanoscale filler was involved in this earlier process.

In one embodiment of the present invention, a matrix-forming material (e.g., thermoplastic powder particles 20 in FIG. 1( b)), in addition to nano-fillers, is also suspended in the fluidized medium. The resulting structure, composed of continuous fibers, nano-fillers, and a matrix-forming material, is hereinafter referred to as a hybrid towpreg. The hybrid towpreg may then be subjected to a weaving, winding, or any other textile structure-forming procedure to produce a composite preform, which is heated (to melt out the thermoplastic powder) and consolidated to obtain a composite structure.

In the presently invented hybridfiber tow approach, the continuous hybrid fiber tow may be directed to enter a resin bath for impregnation with a matrix resin in a filament-winding, prepreg-forming, pultrusion, or fiber placing operation (FIG. 3). This resin can be a thermosetting, cyclic, or thermoplastic resin. This is an automated process that is suitable for mass production of hybrid composites.

Alternatively, the hybrid fiber tow may be subjected to weaving, winding, braiding, stitching, knitting, freeform fabrication, and/or other textile-forming procedures to produce a dry composite preform, which is then impregnated with a matrix material to obtain a composite structure. With a polymer matrix, the preform can be impregnated through resin transfer molding, reaction injection molding, vacuum-assisted transfer molding, pressure-assisted liquid resin impregnation, etc. For a metal matrix, the preform can be impregnated through microwave-assisted infiltration, liquid metal impregnation, etc. For a glass or ceramic matrix, the preform can be impregnated through chemical vapor deposition or chemical vapor infiltration. A resin-impregnated preform can be subjected to a heat treatment (pyrolization) that converts a polymer into a carbonaceous matrix.

The nanoscale filler that can be used in the presently invented hybrid fiber tow, towpreg, or composite can be a nanoscale graphene plate, non-graphite platelet, carbon nano-tube, nano-rod, carbon nano-fiber, non-carbon nano-fiber, or a combination thereof. These entities all have one thing in common—they have at least on elongate axis. For instance, CNTs have one elongate axis (in the tube axial direction) and platelets have two elongate axes (in the length and width direction). The resulting hybrid composite can easily have nano-fillers that are present at a loading of greater than 5% by weight based on the total weight of nano-fillers plus the matrix material. The nano-fillers in many cases exceed 15% by weight. A majority of these nano-fillers have an elongate axis oriented at an angle of at least 45 degrees with respect to the continuous fiber axis. If improved transverse thermal or electrical conductivities are desired, carbon-based nano-fillers are preferred.

The NGPs obtained in our facilities typically have a platelet thickness of 1-100 nm and length and width of 0.1-10 μm. These rigid two-dimensional platelets appear to be conducive to fitting into inter-filament interstices. The nanoscale graphene plate or non-graphite platelet that has a length or width smaller than 500 nm is particularly well-suited to the present application. The flexibility of both the CNT and the CNF afforded to by their large length-to-diameter ratios makes these one-dimensional structures tend to assume curved or coiled shapes and should make it more difficult to be incorporated in a hybrid composite. However, surprisingly, the presently invented process is capable of incorporating CNTs and CNFs into the inter-filament spaces.

A fluidized-bed powder impregnation apparatus, schematically shown in FIG. 2, was designed and constructed for incorporating a controlled percentage of nano-fillers in a continuous fiber tow to produce a hybrid fiber tow. The apparatus is mainly composed of a feeder roller, a fiber tow spreader, a fluidized bed chamber, and an optional fiber tow consolidator. The fiber tow (or strands of fibers or filaments) is reeled from a fiber spool or feeder roller and directed to go through a tow spreader in which individual filaments are separated from each other. The separated fibers are fed into the fluidized bed chamber in which the nano-fillers, “fluidized” by an air flow, are introduced to impinge upon the fibers. The nano-fillers may be electrostatically charged and the fiber tow grounded to promote nano-filler impregnation of the fiber tow, with nano-fillers residing in interstices between fibers while being re-merged or compacted.

The fluidized bed powder coating apparatus are well-known in the art. For instance, these apparatus were successfully used to prepare a towpreg that is composed of reinforcing filaments coated with matrix-forming resin powder as a precursor to a plastic matrix composite [J. Lamanche, et al., U.S. Pat. No. 3,703,396 (Nov. 21, 1972)]. A key component in the system is a tow spreader. Spreading of the filaments can be achieved by vibrating the graphite fiber tow in air pulsating at a frequency and intensity sufficient to couple the energy of the pulsating medium to the graphite tow [e.g., S. Iyer, et al., U.S. Pat. No. 5,042,122 (Aug. 27, 1991)]. Spreading may also be facilitated or promoted by using air currents or electrostatic charges of the same polarity.

An optional filament re-merger or compactor may be used to facilitate the merging of separated filaments, along with the embedded nano-fillers, into a more compact fiber tow. This filament re-merging step can occur before, during, and after the resin impregnation step. Resin impregnation can be part of a filament-winding, prepreg-forming, fiber-placing, or pultrusion process.

The continuous filament can be a polymer fiber, ceramic fiber, carbon fiber, graphite fiber, glass fiber, or a combination thereof. In the hybrid fiber tow, the nano-fillers are preferably oriented in a direction substantially non-parallel to the continuous filament axial direction and further preferably perpendicular to the filament axis.

In summary, the process for producing a hybrid fiber tow comprises (a) spreading a continuous fiber tow into multiple, separated filaments that define interstitial spaces between the filaments; (b) exposing the separated filaments to a fluid medium or fluidized medium containing nanoscale fillers under a flow condition for a duration of time sufficient to cause the nanoscale fillers to stay in the interstitial spaces; and (c) moving the separated filaments with the interstitial nanoscale fillers away from the medium to produce the hybrid fiber tow. The step of exposing can comprise moving the separated filaments through a fluidized bed comprising a fluidized medium that contains the nanoscale particles suspended in the medium, as illustrated in FIG. 2. The fluidized bed may be provisioned with electrostatic charging means to facilitate attraction of the nanoscale fillers to the filaments.

Alternatively, the step of exposing comprises moving the separated filaments through a fluid medium that contains the nanoscale particles suspended in a liquid or solution. In other words, the fluidized-bed powder coater device shown in FIG. 2 is now replaced by a tank of liquid with nano-fillers suspended in the liquid which is driven (e.g., pumped) to flow around in the tank with a desired flow pattern that enables impingement of nano-fillers with the continuous filaments at a desired direction.

Further alternatively, as schematically shown in FIG. 4, the step of exposing can comprise moving the separated filaments at a desired speed in a desired direction while directing a stream of a liquid medium (or gaseous medium such as air) containing the nanoscale fillers to impinge upon the filaments in such a manner that the fillers are trapped in the interstitial spaces to form a hybrid fiber tow. Again, the process begins with reeling a continuous fiber tow 54 from a fiber spool 32, feeding the tow into a tow spreader 56 to obtain separated filaments 58, which are impinged upon by a fluid (liquid or air) suspending nano-fillers 60 therein. The nano-fillers are trapped between filaments, allowing the liquid or air to filter through the gaps between filaments. The hybrid tow 62 can be optionally compacted by a compactor or consolidator 64 to become consolidated hybrid fiber tow which is then collected on a take-up roller 46. Again, such a roll-to-roll process is suitable for mass production of composites.

The fluid medium (e.g., in FIG. 4) or fluidized medium (e.g., in FIG. 2 or 3) can further contain a matrix-forming material. The step of exposing then comprises causing both the nanoscale fillers and the matrix-forming material to stay in the interstitial spaces to form a matrix-forming material-impregnated hybrid tow, referred to as a hybrid fiber towpreg. The matrix-forming material may also be coated onto the surface of continuous filaments.

In another embodiment of the present invention, in addition to the aforementioned steps (a), (b), and (c), the process further comprises (d) reeling the continuous fiber tow from a roller or spool prior to the fiber tow spreading step; (e) impregnating the hybrid fiber tow with a matrix material to form a matrix-impregnated hybrid fiber tow; (f) subjecting the matrix-impregnated hybrid tow to a shape-forming operation to form a composite shape; and (g) consolidating the composite shape through heating, curing, and/or cooling the matrix material to form a hybrid composite structure. The shape-forming operation can comprise a filament winding, fiber placement, prepreg-forming, pultrusion, freeform fabrication step, or a combination thereof. Freeform fabrication involved computerized deposition of a material point-by-point and layer-by-layer. The process is also commonly referred to as rapid prototyping.

In yet another embodiment of the present invention, the process comprises, in addition to aforementioned steps (a), (b), and (c), the following steps: (d) reeling the continuous fiber tow from a roller or spool prior to the fiber tow spreading step; (e) subjecting the hybrid fiber tow to a shape-forming operation to form a composite preform; (f) impregnating the preform with a matrix material; and (g) consolidating the matrix-impregnated preform through heating, curing, and/or cooling the matrix material to form a hybrid composite structure. The shape-forming operation can comprise a step of filament winding, fiber placement, freeform fabrication, weaving, braiding, stitching, knitting, or a combination thereof.

In still another embodiment of the present invention, the process comprises, in addition to the aforementioned steps (a), (b), and (c), the following steps: (d) reeling the continuous fiber tow from a roller or spool prior to the fiber tow spreading step; (e) subjecting the hybrid fiber towpreg to a shape-forming operation to form a composite shape; and (f) consolidating the composite shape through heating, curing, and/or cooling the matrix-forming material to form a hybrid composite structure. The shape-forming operation can include a step of filament winding, fiber placement, prepreg-forming, freeform fabrication, weaving, braiding, stitching, knitting, or a combination thereof.

In all of the aforementioned versions of the invented process, the step of consolidating can comprise melting a matrix material, cooling or solidifying a matrix material, curing a resin, polymerizing or cross-linking a resin precursor, converting an organic or polymeric material to a carbonaceous material, chemical vapor infiltration, or a combination thereof.

As examples to illustrate the utility value of the developed hybrid composites, we obtained the thermal conductivity values of a series of hybrid composites containing continuous graphite fibers (approximately 60% by volume of the total composite) and NGP/epoxy matrix materials (with several NGP weight fractions based on the total NGP/epoxy weight) or CNT/epoxy matrices. As shown in FIG. 5, the transverse thermal conductivity values of hybrid composites (with NGPs substantially perpendicular to the continuous graphite fiber direction) can be improved from approximately 9.1 Wm⁻¹K⁻¹ for a continuous graphite fiber/epoxy composite (0% NGP) to 193 Wm⁻¹K⁻¹ for a hybrid composite (15% NGP). The longitudinal thermal conductivity values, parallel to the continuous graphite fiber direction, for all these composites are in the range of 125-180 Wm⁻¹K⁻¹, relatively independent of the NGP content in the matrix. Addition of CNTs has also significantly improved the transverse thermal conductivity of the hybrid composite, albeit to a smaller extent. The high transverse as well as longitudinal thermal conductivity for these composites is a highly significant result since thermally conductive composites can be used as thermal management materials for microelectronic devices and rocket motor cases, just to cite two of many examples. 

1. A hybrid fiber tow comprising multiple continuous filaments and nanoscale fillers embedded in interstitial spaces between said continuous filaments, wherein said nanoscale fillers comprise a nanoscale graphene plate, non-graphite platelet, carbon nano-tube, nano-rod, carbon nano-fiber, non-carbon nano-fiber, or a combination thereof.
 2. The hybrid fiber tow as defined in claim 1, wherein said nano-fillers comprise a nanoscale graphene plate or a non-graphite platelet with a width or length smaller than 10 μm.
 3. The hybrid fiber tow as defined in claim 1, wherein said nano-fillers comprise a nanoscale graphene plate or non-graphite platelet that has a length or width smaller than 500 nm.
 4. The hybrid fiber tow as defined in claim 1, further comprising a matrix-forming material embedded in interstitial spaces or coated on a surface of said continuous filaments.
 5. The hybrid fiber tow as defined in claim 4, wherein said matrix-forming material comprises a thermoplastic, a thermoset, or a combination thereof.
 6. The hybrid fiber tow as defined in claim 1, wherein said continuous filaments comprise a polymer fiber, ceramic fiber, carbon fiber, graphite fiber, glass fiber, or a combination thereof
 7. The hybrid fiber tow as defined in claim 1, wherein said nano-fillers are preferentially oriented in a direction substantially non-parallel to a continuous filament axial direction.
 8. A hybrid composite structure comprising a hybrid fiber tow as defined in claim 1 and a matrix material.
 9. The hybrid composite as defined in claim 8, wherein said matrix material comprises a polymer, glass, carbon, ceramic, metal, or a combination thereof; said continuous filaments comprise a polymer fiber, ceramic fiber, carbon fiber, graphite fiber, glass fiber, or a combination thereof; and said nano-fillers comprise a nanoscale graphene plate, carbon nano-tube, carbon nano-fiber, or a combination thereof.
 10. A process for producing the hybrid fiber tow of claim 1, said process comprising a) spreading a continuous fiber tow into multiple, separated filaments that define interstitial spaces between said filaments; b) exposing said separated filaments to a fluid medium or fluidized medium containing said nanoscale fillers suspended therein under a flow condition for a duration of time sufficient to cause said nanoscale fillers to be trapped and stay in said interstitial spaces; and c) moving said separated filaments with said trapped interstitial nanoscale fillers away from said medium to produce said hybrid fiber tow.
 11. The process of claim 10, wherein said step of exposing comprises moving said separated filaments through a fluidized bed comprising a fluidized medium that contains said nanoscale particles suspended in said medium.
 12. The process of claim 11, wherein said fluidized bed is provisioned with electrostatic charging means to facilitate attraction of said nanoscale fillers to said filaments.
 13. The process of claim 10, wherein said step of exposing comprises moving said separated filaments through a fluid medium that contains said nanoscale particles suspended in a liquid or solution.
 14. The process of claim 10, wherein said step of exposing comprises moving said separated filaments at a desired speed in a desired direction while directing a stream of a liquid medium containing said nanoscale fillers to impinge upon said filaments in such a manner that said fillers are trapped in said interstitial spaces to form said hybrid fiber tow.
 15. The process of claim 10, wherein said step of exposing comprises moving said separated filaments at a desired speed in a desired direction while directing a stream of a gaseous medium carrying said nanoscale fillers to impinge upon said filaments in such a manner that said fillers are trapped in said interstitial spaces to form said hybrid fiber tow.
 16. The process of claim 10 wherein said fluid medium or fluidized medium further contains a matrix-forming material and said step of exposing comprises causing both said nanoscale fillers and said matrix-forming material to stay in said interstitial spaces to form a matrix-forming material-impregnated hybrid tow, herein referred to as a hybrid fiber towpreg.
 17. The process of claim 10, further comprising a step of reeling said continuous fiber tow from a roller or spool prior to the fiber tow spreading step and a step of winding said hybrid fiber tow on a roller or drum.
 18. The process of claim 16, further comprising a step of reeling said continuous fiber tow from a roller or spool prior to the fiber tow spreading step and a step of winding said hybrid fiber towpreg on a roller or drum.
 19. The process of claim 10, further comprising d) reeling said continuous fiber tow from a roller or spool prior to the fiber tow spreading step; e) impregnating said hybrid fiber tow obtained in step (c) with a matrix material to form a matrix-impregnated hybrid fiber tow; f) subjecting said matrix-impregnated hybrid tow to a shape-forming operation to form a composite shape; and g) consolidating said composite shape through heating, curing, and/or cooling said matrix material to form a hybrid composite structure.
 20. The process of claim 19 wherein said shape-forming operation comprises a filament winding, fiber placement, prepreg-forming, pultrusion, freeform fabrication step, or a combination thereof.
 21. The process of claim 10, further comprising d) reeling said continuous fiber tow from a roller or spool prior to the fiber tow spreading step; e) subjecting said hybrid fiber tow obtained in step (c) to a shape-forming operation to form a composite preform; f) impregnating said preform with a matrix material; and g) consolidating the matrix-impregnated preform through heating, curing, and/or cooling said matrix material to form a hybrid composite structure.
 22. The process of claim 21 wherein said shape-forming operation comprises a step of filament winding, fiber placement, freeform fabrication, weaving, braiding, stitching, knitting, or a combination thereof.
 23. The process of claim 16, further comprising d) reeling said continuous fiber tow from a roller or spool prior to the fiber tow spreading step; e) subjecting said hybrid fiber towpreg to a shape-forming operation to form a composite shape; and g) consolidating said composite shape through heating, curing, and/or cooling said matrix-forming material to form a hybrid composite structure.
 24. The process of claim 23 wherein said shape-forming operation comprises a step of filament winding, fiber placement, prepreg-forming, freeform fabrication, weaving, braiding, stitching, knitting, or a combination thereof.
 25. The process of claim 19 wherein said step of consolidating comprises melting a matrix material, cooling or solidifying a matrix material, curing a resin, polymerizing or cross-linking a resin precursor, converting an organic or polymeric material to a carbonaceous material, or a combination thereof.
 26. The process of claim 21 wherein said step of consolidating comprises melting a matrix material, cooling or solidifying a matrix material, curing a resin, polymerizing or cross-linking a resin precursor, converting an organic or polymeric material to a carbonaceous material, or a combination thereof.
 27. The process of claim 23 wherein said step of consolidating comprises melting a matrix material, cooling or solidifying a matrix material, curing a resin, polymerizing or cross-linking a resin precursor, converting an organic or polymeric material to a carbonaceous material, or a combination thereof.
 28. The hybrid composite of claim 8 wherein said nano-fillers are present at a loading of greater than 5% by weight based on the total weight of nano-fillers plus the matrix material.
 30. The hybrid composite of claim 8 wherein said nano-fillers are present at a loading of at least 15% by weight based on the total weight of nano-fillers plus the matrix material.
 30. The hybrid composite of claim 8 wherein said nano-fillers have an elongate axis that is inclined at an angle of at least 45 degrees with respect to a longitudinal axis of said continuous fibers. 